The present invention relates to a connection admission control method in an ATM network system and, more particularly, to a connection admission control method for determining whether or not a connection is to be accepted on the basis of the average cell rate, the peak cell rate and the bandwidth of a transmission link which are declared by the user at the time of calling.
There is increasing demand not only for audio communication and data communication but also for multimedia communication in which moving pictures are transmitted as well as audio and data. As a means for realizing such broadband communication, an agreement has been reached by the ITU-T upon an exchanging technique in a B-ISDN (Broadband-ISDN) system, which is based on an asynchronous transfer mode (ATM), and the technique is being put to practical use.
In the ATM, all the information is converted into fixed information which is called a cell without depending upon continuous information such as an audio and a moving picture or burst information such as data, and transferred at high speed without depending upon the respective communication speed. More specifically, in the ATM, a line is allocated to a plurality of calls by establishing a multiplicity of logical links on a physical line. The moving picture data and the audio data transmitted from a terminal corresponding to each call are separated into information units (which are called "cells") having a fixed length, and the cells are serially transmitted over a line, thereby realizing multiplex communication.
Each cell is composed of a block having a fixed length of 53 bytes, as shown in FIG. 26. In the 53 bytes, 5 bytes constitute a header portion HD and 48 bytes an information field (information portion) DT. The header portion HD includes a virtual channel identifier (VCI) for identifying a connection so as to indicate the destination even after the data is separated into blocks. The header portion HD also includes a virtual path identifier (VPI) for specifying a path, a generic flow control (GFC) which is used for flow control between links, a payload type (PT), a header error control (HEC) for correcting errors, etc.
FIG. 27 schematically shows the structure of an ATM network so as to explain ATM transmission. In FIG. 27, the reference numerals 1a, 1b represent ATM terminals, and 3 an ATM network. The ATM network 3 is provided with an information network 3a for transferring a data cell, and a signal network 3b for transferring a control signal. The call processing units or processors (CPUs) 3d-1 to 3d-n of the ATM network systems 3c-1 to 3c-n in the information network 3a are connected to the signal network 3b.
When the originating terminal 1a executes a calling operation so as to call the terminating terminal 1b, the cell assembling portion of the originating terminal partitions the SET UP message (data which includes the originating number, the terminating number, the type of terminal, the average cell rate, the peak cell rate, etc.) into cell units, attaches a signal VCI (which is determined in advance for the respective terminal) to each partitioned data to form a signal cell and sends the signal cell to the ATM network 3.
When the signaling device of the ATM network system 3c-1 (on the originating side) receives the signal cell, it assembles the information contained in the signal cell and supplies the assembled information to the CPU 3d-1. On the basis of the received message, the CPU executes such calling processing as processing for analyzing calling-party service, billing processing, processing for interpreting digits on the side of the terminating party, etc., and determines a virtual path identifier (VPI) and a virtual channel identifier (VCI) on the basis of the declared average cell rate and peak cell rate, and in accordance with a protocol No. 7, supplies connection information which includes data such as the originating umber, terminating number, VPI and VCI, to the subsequent relay exchange 3c-2 via the signal network 3b. The relay exchange 3c-2 executes similar processing to that of the exchange 3c-1 on the originating side. After repetition of similar processing, the paths and the relay ATM network systems 3c-2, 3c-3, . . . between the exchange 3c-1 on the originating side and the ATM network system 3c-n to which the terminating terminal 1b is connected are finally determined. When the ATM network system 3c-n on the terminating side receives the connection information including the originating number, the terminating number and the VCI of the higher-order ATM network system 3c-3, the ATM network system 3c-n allocates a predetermined VCI to the terminating terminal 1b and judges whether or not the terminating terminal 1b is communicable. If the answer is YES, the signal network 3b informs the exchange 3c-1 on the originating side that the terminating terminal 1b is communicable, and the exchange 3c-1 on the originating side allocates a predetermined VCI to the originating terminal 1a.
Each of the ATM network systems 3c-1 to 3c-n on paths registers the following, for each path, in an internal routing table in a form correlated with the VCI of the higher-order ATM network system: (1) information (referred to as routing information or tag information) for specifying the outgoing highway of the cell having the specific VCI, and (2) new VCI and VPI which are to be added to the outputted cell.
In this manner, when the paths are formed between the originating terminal 1a and the terminating terminal 1b, these terminals 1a, 1b transmit and receive the signaling cells and the response cells and confirm the communication procedure in mutual fashion. Thereafter, the originating terminal 1a separates the data to be transmitted into predetermined byte lengths, generates a cell with a header including the allocated VCI attached thereto, and sends the cell to the ATM network 3. When the cell is input from the higher-order exchange through a predetermined incoming highway, each of the ATM network systems 3c-1 to 3c-n replaces the VPI/VCI of the input cell by reference to its own routing table and sends the cell out on a predetermined outgoing highway on the basis of the tag information. As a result, the cell output from the originating terminal 1a reaches to the exchange 3c-n on the terminating side via the paths determined by the call control. The exchange 3c-n on the terminating side replaces the VCI which is attached to the input cell with the VCI allocated to the terminating terminal 1b by reference to its routing table and sends the cell to the line to which the terminating terminal 1b is connected.
Thereafter, the originating terminal 1a serially transmits the cells to the terminating terminal 1b, and the terminating terminal 1b assembles the information portion DT contained in the received cells and restores the original data.
In the above explanation, only one call is processed, but by providing different VCI values for both ends of the respective lines between a terminal and an ATM network system and between mutually adjacent ATM network systems, it is possible to establish logical links on one line in correspondence with a multiplicity of calls, thereby realizing high-speed multiplex communication. According to the ATM, it is possible to multiplex information from information sources having different transmission rates such as moving pictures, data and audio, so that a single transmission line can be effectively used. In addition, data transmission at a very high speed on the order of 150 Mbps to 600 Mbps is enabled without the need for retransmission control or a complicated communication procedure which is conventionally implemented by software through packet switching.
An ATM network system has a buffering function, which enables the ATM network system to accept a call without keeping the originating terminal waiting and to send it to the terminating terminal even if there are a multiplicity of calls to the ATM network system or the terminating terminal. For example, when there are a multiplicity of simultaneous calls to the terminating terminal 1b and therefore there is no vacant line between the exchange 3c-n on the terminating side and the terminating terminal 1b, there remains a cell which cannot be sent to the terminating terminal 1b. In this case, the exchange 3c-n on the terminating side buffers the remaining cell and sends it when a line becomes vacant. In this manner, it is possible to accept a call to the terminating terminal without keeping the originating terminal waiting.
FIG. 28 shows the structure of a self-routing ATM network system. The self-routing ATM network system is provided with a basic switching unit SWU, a control information add-on unit CIAU, and a CPU (call processing unit) for processing a calling. Although one self-routing switch module SRM 1 exists between the input lines and the output lines in this ATM network system, a plurality of self-routing switch modules may be connected between them.
The input ends of the module SRM 1 are connected to the input lines (incoming highways) #1 to #3 via the control information add-on unit CIAU, and the output ends are connected to the output lines (outgoing highways) #1 to #3. The control information add-on unit CIAU is provided with add-on circuits AC1 to AC3 for adding routing information or the like in correspondence with the respective input lines #1 to #3. Each of the add-on circuits AC1 to AC3 adds a tag (routing header) to the cell which is input from the corresponding input line, replaces the VCI contained in the cell information and supplies the cell to the basic switching unit SWU.
The call processing unit CPU controls a call so as to determine the VCI and the VPI of a cell at the time of calling, determines the routing header RH in accordance with the location of the terminating terminal and writes the control information (VPI/VCI, RH) in a routing table (not shown) of the add-on circuit to which the cell is input. The add-on circuit to which the cell is input is already clear from the information supplied by a higher-order ATM network system. That is, the determined control information is written in the routing table of the add-on circuit in correspondence with the VCI of the higher-order ATM network system.
When the cell is input to a predetermined input line via the higher-order ATM network system after the end of the call control, one of the add-on circuit AC1 to AC3 which is connected to the input line reads, from the routing table, the control information (routing header RH and VCI) which corresponds to the VCI attached to the input cell. The add-on circuit attaches the routing header RH to the cell, replaces the VCI of the cell with the read VCI, and supplies the cell to the basic switching unit SWU.
The self-routing switch module SRM 1 of the basic switching unit SWU transmits the cells from a predetermined output line in accordance with the routing header RH. The routing header is removed by a post-processing circuit (not shown) before the cell is transmitted to the output line.
FIG. 29 is a circuit diagram of an example of the self-routing switch module (SRM 1). The symbols I.sub.1 to I.sub.3 represent control information detectors, D.sub.1 to D.sub.3 transmission information delay circuits, DM.sub.1 to DM.sub.3 demultiplexers, and DEC.sub.1 to DEC.sub.3 control information decoders. All these elements constitute a cell distributor CELD. The symbols FM.sub.11 to FM.sub.33 represent buffer memories such as FIFO (First-In First-Out) memories, SEL.sub.1 to SEL.sub.3 selectors, and AOM.sub.1 to AOM.sub.3 arrival-order management FIFOs. The arrival-order management FIFOs (AOM.sub.1 to AOM.sub.3), which are connected to the output ends of the control information decoders DEC.sub.1 to DEC.sub.3, respectively, store the order of arrival of the cells into the corresponding three buffer memories FM.sub.11 to FM.sub.13, FM.sub.21 to FM.sub.23 and FM.sub.31 to FM.sub.33, respectively, control the selectors SEL.sub.1 to SEL.sub.3, respectively, so as to read the cells from the buffer memories in the order of arrival, and supply the cells to the output lines #1 to #3, respectively.
The detector I.sub.i (i=1 to 3) extracts the control information contained in the cell and supplies the information to the decoder DEC.sub.i (i=1 to 3).
If the input routing header RH represents the output terminal #j (j=1 to 3), the decoder DEC.sub.i operates the demultiplexer DM.sub.i by a switch signal S.sub.i and transmits the transmission information to the FIFO memory FM.sub.ji. For example, if the routing header RH contained in the information input from the input terminal #1 represents the output terminal #2, the decoder DEC.sub.1 operates the demultiplexer DM.sub.1 and inputs the information supplied from the input terminal #1 to the FIFO memory FM.sub.21. The arrival-order management FIFO (AOM.sub.i) is connected to the output terminal of the corresponding control information decoder DEC.sub.1 to DEC.sub.3 and stores the order of arrival of the cells to the corresponding three buffer memories FM.sub.i1 to FM.sub.i3. For example, if the cells arrive to the buffer memories in the order of FM.sub.11, FM.sub.12, FM.sub.13, FM.sub.12, . . . , buffer memory identification codes 1, 2, 3, 2, . . . are stored in the arrival-order management FIFO (AOM.sub.i) in the order of arrival of the cells. Thereafter, the arrival-order management FIFO (AOM.sub.i) controls the corresponding selector SEL.sub.i, to read the cells from the three buffer memories FM.sub.i1 to FM.sub.i3 in the order of arrival of the cells and supplies the cells to the output line #i.
In this manner, since the FIFO memory FM.sub.ij has a capacity for a plurality of cells, it has a buffering function which is capable of adequately dealing with the problem such as a temporary increase of transmission data. In addition, since cells are read from the buffer memories FM.sub.i1 to FM.sub.i3 in the order of arrival of the cells, an equal number of cells remain in each of the buffer memories FM.sub.i1 to FM.sub.i3, and it never happens that cells overflow a buffer memory and are therefore discarded.
The ATM transmission, however, has the following problem. Since various traffics having different information rates and different burst properties (burst means an abrupt increase in the quantity of information) are synthetically handled, when there is a traffic having an especially strong burst property, it is impossible without an appropriate call reception control to provide a service quality (cell loss ratio, delay time) which is required by the user. For this reason, in the case of a bandwidth-guaranteed connection call, an ATM network system judges whether or not a necessary bandwidth is vacant in a predetermined transmission line on the basis of the physical bandwidth of the transmission line, the average cell rate and the peak cell rate which are declared from the user (ATM terminal), and if the answer is in the affirmative, the ATM network system accepts the call, while rejecting the call if the answer is in the negative. If there is a call having a variable-speed traffic property in which the average cell rate is different from the peak cell rate, adoption of a controlling method which determines whether or not the call is accepted on the assumption that the peak cell rate of the call is a necessary bandwidth is simple, but it reduces the number of calls which can be allocated to a transmission line, thereby lowering the utilization of a transmission line. On the other hand, a controlling method which determines whether or not the call is accepted on the assumption that the necessary bandwidth is the average cell rate can allocate many calls to a transmission line, thereby enhancing the utilization of a transmission line. However, if cells are allocated on the basis of the average cell rate, for example, when the peak of the transmission rate for each call overlaps one another, cells beyond the bandwidth of the transmission line are lost. As a result, it is impossible to meet the required cell loss ratio, which causes sound skipping, picture missing and data loss on the terminating side. In order to solve these problems, cells are allocated on the basis of both the average cell rate and the peak cell rate in the call reception control adopted at present, thereby enhancing the utilization of a transmission line while maintaining a predetermined cell loss ratio.
FIG. 30 is an explanatory view of the connection admission control in an ATM network system, and FIG. 31 is a flow chart of the connection admission control algorithm of a conventional ATM network system. In FIG. 30, (1) the reference numeral Vt represents the physical bandwidth of a transmission line, (2) Vpht the sum of the peak (maximum) cell rates of all the calls allocated on the basis of the peak cell rate, (3) Vavt the sum of the average cell rates of all the calls allocated on the basis of the average cell rate, and (4) Vpts the sum of the peak cell rates of all the cells that are in the process of communication. Further, (5) the reference numeral Vp represents the peak cell rate of a new call (which is requesting admission), and (6) Vav the average cell rate of the call which is requesting admission.
When the number of calls allocated to the transmission line increases, the peaks and the bottoms of the transmission rates overlap each other and they are levelled due to a statistical multiplex effect, so that it is possible to accommodate a larger number of calls than an apparent number of calls. In the connection admission control (CAC) shown in FIG. 31, calls are allocated on the basis of both the average cell rate and the peak cell rate by utilizing such a statistical multiplex effect.
When a user requests a call to be accepted (at the time of calling), the user declares the parameters (peak cell rate (Vp), average cell rate (Vav), burst duration, number (Nc) of cells during the burst duration, burst occurrence interval, burst ratio (Rb), etc.) indicating the attribute of the call to the ATM network system. The ATM network system calculates the average cell traffic a, the distribution v of the cell traffic, and the coefficient v/a of call fluctuation from the declared parameters (step 101). If the coefficient v/a of call fluctuation is not more than 1, the ATM network system allocates the call on the basis of the average cell rate, while if the coefficient v/a is more than 1, the ATM network system allocates the call on the basis of the peak cell rate.
It is also possible to determine whether the call requesting admission is allocated on the basis of the average cell rate or the peak cell rate depending on whether (Vt-Vpht)/Vp is not less than a preset value X, wherein X represents the minimum (transmission rate/maximum information cell rate) when burst multiplex traffic can be approximated in accordance with a Poisson distribution, and X is, for example, 100. More specifically, how many (Y) peak cell rates Vp of the call which is requesting admission are accommodated in the bandwidth (Vt-Vpht) obtained by subtracting the sum Vpht of the peak cell rates of all the calls that are allocated on the basis of the peak cell rate from the capacity Vt of the transmission line is calculated. If Y .gtoreq.X, the peaks and the bottoms of the transmission rates overlap each other and they are levelled due to a statistical multiplex effect, so that it is possible to accommodate a larger number calls than an apparent number of calls. In this case, the call is allocated on the basis of the average cell rate. On the other hand, if Y &gt;X, the call is allocated on the basis of the peak cell rate (step 102).
When the call is allocated on the basis of the average cell rate, it is then judged whether or not the call can be allocated in the vacant bandwidth of the current transmission line. That is, judgement is made as to whether or not Vav+Vavt is smaller than (Vt-Vpht).multidot..rho.max (step 103), wherein .rho.max represents the maximum utilization factor determined by a prescribed cell loss ratio CLR and the amount m of buffer when a M/D/1 model is used. The M/D/1 model is a switch module in which cells reach the buffer at random from a plurality of input lines and are read out of the buffer at regular intervals from one output line.
If (Vav+Vavt).ltoreq.(Vt-Vpht).multidot..rho.max, since the call can be allocated in a vacant bandwidth of the current transmission line, Vpts and Vavt are renewed by Vpts+Vp .fwdarw.Vpts, Vavt+Vav.fwdarw.Vavt (step 104), the call is accepted and the next call is waited for. On the other hand, if (Vav+Vavt)&gt;(Vt-Vpht).multidot..rho.max, since it is impossible to accept the call in a vacant bandwidth of the current transmission line, the admission of the call is rejected (step 105) and the next call is waited for.
When the call is allocated on the basis of the maximum rate at the step 102, it is judged whether or not the call can be allocated in a vacant bandwidth of the current transmission line. That is, judgement is made as to whether or not (Vpts+Vp).ltoreq.Vt (step 106), if the answer is YES, since the cell can be allocated in a vacant bandwidth of the current transmission line, Vpts and Vpht are renewed by Vpts+Vp.fwdarw.Vpts, Vpht+Vp.fwdarw.Vpht (step 107), the call is accepted and the next call is waited for. On the other hand, if (Vpts+Vp)&gt;Vt, since it is impossible to accept the call in a vacant bandwidth of the current transmission line, the admission of the call is rejected (step 105) and the next call is waited for.
In the conventional call admission algorithm, however, since the value of X or v is generally large in the case of burst traffic, approximately all the calls having a burst traffic property are allocated on the basis of the peak cell rate, which brings about a problem of a reduction in the utilization of a transmission line. For example, If it is assumed that X=100 and Vt=150 Mb/s, it is only the calls having a peak cell rate of not more than 1.5 Mb/s that the calls can be allocated on the basis of the average cell rate, and all the calls having a variable-speed traffic property such as the calls having a peak cell rate of 10 Mb/s, 20 Mb/s, or the like are allocated on the basis of the peak cell rate, which reduces the utilization of the transmission line.